Note. On models equipped with traction control (Traction Control), ABS performs two functions: anti-lock braking system, as well as the functions of the automatic stabilization system and traction control (ASC+T).
1. ABS is installed on most models as the main equipment, on all other models it can be installed by request. The system contains a hydraulic unit that includes hydraulic solenoid valves, an electrically driven return pump, four wheel sensors (one for each wheel) and electronic control unit (BEU). The system is designed to prevent the wheels from locking during heavy braking. This is done by automatically reducing the braking force on the respective wheel and then increasing the force.
2. Solenoid valves receive a control signal from the BEU, which, in turn, receives signals from four wheel sensors that control the speed of rotation of each wheel. Based on the results of comparing these signals, the BEU determines the speed of the vehicle. This speed value is used by the BEU to determine when one of the wheels decelerates to too low a speed compared to the speed of the car and thus determines which wheel is in danger of blocking. In normal mode, the system operates in the same way as a non-ABS braking system.
3. When the BEU determines that one of the wheels is in danger of blocking, it issues a signal to the corresponding valve of the hydraulic block. This valve cuts off the wheel brake caliper that is in danger of blocking. from the main cylinder, blocking the pressure supply to it from the hydraulic system.
4. If the wheel speed continues to decrease to an excessively low value. BEU includes a return pump. which pumps the brake fluid back to the master cylinder. Due to this, the pressure in the brake caliper is reduced so much that the wheel is released. As soon as the wheel speed increases again to an acceptable value. the pump stops and the hydraulic valve opens, connecting the brake caliper to the master cylinder. As a result, the brake is re-engaged. This cycle can be repeated up to 10 times per second.
5. The operation of the valves and the return pump creates pulsations in the hydraulic circuits. During ABS operation, such pulsations can be felt» on the brake pedal.
6. The operation of the ABS is completely dependent on the transmission of electrical signals. To prevent the system from triggering from abnormal signals, a safety block is built into it, which controls all signals received by the BEU. When an abnormal signal is received or a low battery voltage is detected, the ABS will automatically turn off and the ABS fault indicator will illuminate on the instrument panel. In this case, normal braking is carried out without the participation of ABS.
7. If an ABS malfunction occurs, the car must be handed over to dealers for diagnostics and repair.
8. On models equipped with ASC+T. a hydraulic accumulator has been added to the hydraulic system. The operation of the ABS system is similar to that described above, and in addition, the ASC + T function has been added to it. If, as the vehicle accelerates, one of the wheels accelerates to an abnormally high speed, the appropriate brake is applied to prevent that wheel from spinning. If a malfunction occurs in the ASC+T system, contact a BMW dealer.